2017 | Nina Bassuk, PhD, Cornell University
The production of diverse, highly desirable tree species is limited by transplanting success. Successful transplanting of nursery stock is determined by a tree’s ability to respond to transplant shock, a function of disrupted and reduced water uptake. While multiple factors including species, plant size, xylem anatomy, root anatomy, and root morphology may influence the efficiency of water uptake, root morphology is one component that can be artificially manipulated in the nursery. Purposeful root pruning was once a common nursery practice that fell out of fashion with the advent of the tree spade. Root pruning, a method of severing tree roots with the aim of promoting root regeneration at or near the wound site, typically results in increased root branching. It is unknown, however, if these newly emerged roots have greater hydraulic conductance efficiency. We plan to manipulate root growth to increase the rate and efficiency of water uptake, resulting in a production practice that can increase a tree’s ability to respond to transplant shock, resulting in greater transplant success and ultimately greater tree species diversity in the nursery industry.
Many highly desirable tree species are often not utilized because they exhibit reduced transplant success. Root pruning a year prior to transplanting, with the aim of promoting new root growth, increased root branching, and to elicit a denser root ball, is one technique that has traditionally been used to increase transplant success with taxa that are considered difficult to transplant. For some species, this technique is effective; however, the physiological mechanism explaining this phenomenon is poorly understood. Water balance of trees is known to be a critical function of transplant success and it relies on roots for water uptake. We questioned if newly-emerged roots arising from pruning wounds exhibit different rates of water uptake compared to unpruned roots, potentially explaining why trees pruned a year before transplanting are able to transplant more readily. To answer this question, an experiment was conducted that evaluated growth and efficiency of water uptake in fine roots of trees that were root pruned the year prior to transplanting and those that were left unpruned. The study included three species known to be difficult to transplant (Nyssa sylvatica, Quercus coccinea, and Quercus macrocarpa), as well as three species that transplant well (Carpinus caroliniana, Gymnocladus dioicus, and Quercus bicolor). Root hydraulic conductance-the rate of water uptake-was measured on fine roots that were sampled in the spring, summer, and fall of the year of transplanting using a hydraulic conductance flow meter. At the conclusion of the season, leaf surface area and caliper growth were measured. Root pruning resulted in an increase in the leaf surface area of Carpinus caroliniana, Gymnocladus dioicus, Quercus bicolor, and Quercus macrocarpa. Caliper was unaffected by root pruning. Specific root hydraulic conductance was not significantly different from controls; however, a clear trend of increased hydraulic conductivity in newly emerged roots as a result of root pruning was observed. This research indicates that the practice of root pruning may benefit nursery stock and reduce the effects of transplant shock and that the concept of hydraulic efficacy of roots emerging from pruning wounds deserves further investigation.